Liquid-crystal modulators have become commercially widely known, particularly in two-dimensional display applications such as wrist watches and flat-screen displays. In most of its applications, a liquid-crystal modulator is in fact a polarization converter which somehow affects the polarization of light incident upon it. Other optical components are used to present the correct light polarization to the liquid-crystal modulator and then to filter out the undesired polarization components. In its most commercially popular form, the liquid-crystal modulator uses a twisted nematic liquid crystal. Alignment layers applied to the two electrodes sandwiching the liquid crystal cause the liquid crystal to twist 90.degree. when the electrodes are in the unbiased state. The twist may be an odd multiple of 90.degree. and have small angular increments to account for other effects. Light incident upon the twisted liquid crystal are waveguided along the twisting liquid crystal, whereby the polarization of the light changes from one side to the other of the liquid crystal cell. However, if a sufficiently high voltage is applied to the electrodes, the twisted waveguiding structure in the liquid crystal is destroyed, and the polarization of light traversing the cell is maintained essentially constant. Thus, an electrical signal applied to the liquid-crystal cell modulates the polarization of light transmitted through the cell.
Patel and Silberberg have disclosed in U.S. Pat. Nos. 5,414,540 and 5,414,541, incorporated herein by reference in their entireties, that liquid-crystal devices can be used to switch individual channels of a multi-wavelength signal, such as is common with an optical wavelength-division multiplexed (WDM) communication network. In such a WDM network, multiple data channels are impressed upon separate lasers or other light sources to produce multiple optical signals having different data signals and different optical carrier wavelengths. By various means, the different optical signals of differing optical wavelength are then impressed upon a single optical channel, such as an optical fiber now very commonly used in telecommunication systems. Thereby, multiple data channels are conveyed along a single optical path.
One of the most fundamental elements in a high-speed telecommunications network is an add/drop multiplexer (ADM). By various means, multiple signals are impressed upon a single physical channel, whether wire, coaxial cable, or fiber. In general, the multiplexing may assume different forms, such as time-division multiplexing, wavelength-division multiplexing, and others. As the physical channel passes through various intermediate nodes in the network, an ADM at that node must be able to extract one or more of the signals multiplexed on the channel and reinsert onto the channel a substitute signal without affecting the other signals not associated with that node.
Wavelength-division multiplexed optical systems have multiple-wavelength signals impressed on a single optical fiber, and an optical ADM for use in a WDM network must be able to extract from the fiber one or more signals at respective wavelengths and to impress upon the fiber other signals at those same wavelengths. In the '540 patent, Patel and Silberberg have disclosed a liquid-crystal add/drop multiplexer 8, illustrated in FIG. 1. This figure is meant to be explanatory only and does not necessarily accurately represent the optical paths or placement of elements. The signal received from the network fiber is designated IN and the signal transmitted to the fiber is designated OUT. The signal extracted from the fiber is designated DROP, and the signal impressed upon the fiber is designated ADD. The fibers are not illustrated in FIG. 1, and other well known and fairly simple optical components couple the free-space and bulk-optics optical paths illustrated in FIG. 1.
The IN and ADD signals are received on optical paths 10, 12 that are incident upon a frequency-dispersive element 14 such as a grating. The frequency-dispersive element 14 spatially divides each of the multiple-wavelength signals on the IN and ADD paths 10, 12 into multiple and separate signals having respective wavelengths and respective paths 16, 18. Note that FIG. 1 illustrates the paths 16, 18 for only one wavelength. The paths for the other wavelengths are arranged in the perpendicular direction, that is, into the plane of the illustration. The optical processing to be described hereafter is performed in parallel for the multiple wavelengths.
The illustrated embodiment is designed to be insensitive to polarization of the input signals. Especially on fiber communication lines, it is nearly impossible to control the signal polarization, which may be changing over time due to environmental and other conditions. The frequency-separated signals on paths 16, 18 strike a first polarization-dispersive element 20, such as a block of properly oriented calcite crystal, which spatially separates each of the signals according to two perpendicular linear polarizations. However, the relative positions of the grating 14 and calcite crystal 20 are not clearly defined in the cited patents. Half-wave plates 21, 22 are placed in the path of the beams output from the calcite crystal 20 and having a first polarization. The half-wave plates 21, 22 thus convert those signals to the perpendicular second polarization. As a result, the two polarization components of both the IN and ADD signals are converted to respective signals that have the same polarization and are spatially separated.
Which polarization is rotated is not of primary importance. A third half-wave plate 24 is placed in the path of both parts of one of the signals, here the ADD signals. As a result, the two beams of the ADD signal are made to have a single polarization perpendicular to the polarization of the two beams of IN signal. It is appreciated that the number of half-wave plates can be reduced to two by combining the effects of plates 21, 24.
A lens 26 focuses all four beams toward a second polarization-dispersive element, preferably a Wollaston prism 28, which has the characteristic, in overly simplified language, that two beams of perpendicular polarization entering the prism 28 at the correct angles are spatially combined into a single beam.
The beam then strikes a segmented liquid-crystal array 30. A segmented liquid-crystal array 30 is similar to a standard twisted nematic polarization converter, but one of its electrodes is divided into multiple sub-electrodes, each separately controlled by respective electrical control signals. The beam illustrated in FIG. 1 strikes one of the segments. Beams corresponding to optical signals of different carrier wavelength (frequency) strike other segments and are each separately controlled.
Depending upon whether the electrical signal applied to the liquid-crystal segment of the wavelength illustrated in FIG. 1 is active or inactive, the IN and ADD signals both either pass through the liquid-crystal cell 30 with their polarizations unchanged or with their polarizations rotated by 90.degree.. If their polarizations are rotated by 90.degree., the effect is to interchange the polarizations of the IN and ADD signals.
After exiting the liquid-crystal modulator 30, the two signals pass through another Wollaston prism 32, a lens 34, half-wave plates 36, 40, 42, a polarization-dispersive element 44, and a frequency-dispersive element 46, all arranged symmetrically to corresponding elements on the other side of the liquid-crystal cell 30. Thereby, the optical operations performed on the input side are undone on the output side. The result is that one set OUT of WDM signals is carried on one output path 48, and another set DROP is carried on another output path 50. Which signal is on which output path 48, 50 is determined by the states of the segments in the segmented liquid-crystal modulator 30. Equivalently stated, the liquid-crystal modulator 30 controls the switching from the ADD and IN signals to the DROP and OUT signals on a wavelength-by-wavelength basis.
The layout and optical paths can be made symmetric about a mirror plane passing through the liquid-crystal modulator 30. It thus becomes possible, as has been explained in the '540 patent to replace all the elements on the output side with a mirror that redirects the beams back through the elements on the input side, thus undoing the effects except for the liquid-crystal polarization modulation. By translating the input beams slightly away from the optical axis, the output beams will be translated in the opposite direction so that they can be separated from the input beams. This translation is most easily accomplished by slightly tilting one of the optical elements, such as the mirror.
The structure of the liquid-crystal modulator 30 is more definitely illustrated in the isometric view of FIG. 2. An unillustrated planar, semi-transparent electrode is formed on the back face of the cell and is powered, typically grounded, through lead 52. A plurality of semi-transparent segmented electrodes 54 are formed on the front face of the liquid-crystal cell and extend in the illustrated vertical direction but are electrically isolated in the horizontal direction. Separate leads 56 supply selective biasing signals to each of the respective segmented electrodes 56. A twisted nematic liquid crystal fills most of the gap 59 between the segmented electrodes 54 and the unillustrated back electrode. The schematic illustration shows neither the two alignment layers to differentially orient the liquid crystal at 90.degree. at the two sides nor the usual glass supports on each side.
Respective optical beams 58 strike the different electrode segments 54. In particular, the one beam illustrated in FIG. 1 strikes one of the electrode segments 54. Other unillustrated beams strike the other electrode segments 54. In a WDM add/drop multiplexer, the different beams 58 striking the respective segments 54 carry separate signals on different WDM optical carrier wavelengths.
An add/drop multiplexer is usually associated with an electronic switch sending and receiving the ADD and DROP signals. However, an optical ADM can also be used as a cross-connect between two optical networks. A simple optical communications network, illustrated by the network diagram of FIG. 3, includes two fiber rings 82, 84, each having multiple nodes 86 associated with different users or entries to other paths. Absent further circuitry, the nodes 86 of each ring 82, 84 can communicate only with the nodes of that ring 82, 84. However, a multi-wavelength optical cross-connect 88 interconnects the two rings 82, 84 and selectively switches signals between the rings 82, 84 on a wavelength-by-wavelength basis, thus enabling wavelength-selective communication between any of the nodes 86 on both rings 82, 84.
The liquid-crystal optical switch 8 of FIG. 1 can beneficially be used as the selective cross-connect 88 for a WDM network. For example, the IN and OUT signals can be associated with one ring 82 and the ADD and DROP signals with the other ring 82. The liquid-crystal switch 8 of FIG. 1 further automatically provides the characteristic required for a WDM cross-connect in most WDM networks that, if a signal at a given wavelength .lambda..sub.i is switched from the first ring 82 to the second ring 84, then another signal at the same wavelength .lambda..sub.i is switched from the second ring 84 to the first ring 82.
The ring topology has enjoyed great use in fiber communications networks because it allows the networks to be made survivable or self-healing if two counter-rotating fibers are used in each ring. By counter-rotating, is meant that two parallel fibers propagate optical signals in anti-parallel directions. As illustrated in the network diagram of FIG. 4, each ring 82, 84 includes one fiber 90 rotating in one direction and another fiber 92 rotating in the other direction. Different ring architectures are possible, but a simple one is that the first fiber 90 is the working fiber and the second fiber 92 is the protection fiber. Each ring 82, 84 is survivable because, if the working fiber 90 is cut at any point (and it is assumed that the protection fiber 92 is also cut at the same point), the two nodes 86 bracketing the break (or protection switches associated with the nodes) will switch all traffic from the working fiber 90 to the counter-rotating protection fiber 92, thereby avoiding the break but retaining full connectivity between all the nodes 86 on the ring 82, 84. The double-fiber ring even protects against the failure of a single node 86. The remaining nodes 86 remain in communication with each other.
A two-ring double-fiber network can be interconnected through two cross-connects 88a, 88b. One cross-connect 88a is connected between the two working fibers 90 and the other crossconnect 88b is connected between the two protection fibers 92. The same interconnection scheme applies to other ring architectures. For example, all signals are broadcast on both fibers 90, 92 and the strongest received signal is used on the receiving end. For almost all ring architectures, the two cross-connects 88a, 88b are maintained in the same switching states.
Of course, the liquid-crystal switch 8 of FIG. 1 can be used for each of the cross-connects 88a, 88b if the two liquid-crystal switches 8 are similarly controlled.
However, any optical switching equipment is expensive to buy and maintain. It is greatly desired to simplify it.
It is further desired to design an optical switch that is compact, efficient, and usable in a telecommunications network.